328 Electric Welding Processes. either case by one of two methods — (i) from an 'alternating' dynamo, the ' current ' being increased by passing through a transformer; (2) from storage or secondary batteries, which take their energy from continuous dynamos. The welding apparatus is not thereby altered. A general diagram in Fig. 308 shews the direct method combined with the Thomson process, where A is the dynamo, B the transformer, and c the welding apparatus. Two wires are clamped in position at D, and end pressure put on by the screws, the current switched on at E and regulated at F. The ends of the wires are previously brightened, and a flux of powdered borax interposed. After welding, the bar or wire is removed and hammered to size. Energy remaining the same, the following examples will shew the variation in ratio of potential and current for various purposes : — 1. For arc lighting : 2500 volts at 2. For incandescent lighting : 100 volts at 3. For welding: 4. For welding : 10 amperes. 250 amperes. volt at 50,000 amperes. volt at 100,000 amperes.* No. 3 would weld steel bars ij inches in diameter in less than two minutes, while No. 4 would do the same in one minute, ab- sorbing 35 H.P., but only for a short time. The great advantage of electric welding lies in the local character of the heating, which prevents the spoiling of a finished piece of work. We will now turn to the Benardos process, shewn in Fig. 309. It is there worked by accumulators — the method most preferred. The batteries being charged from a shunt-wound dynamo, they are connected to a switchboard A, so arranged as to throw them out in sets of five. From this board the current passes through resistance coils for 'farther regulation, and then through the welding tool B; the pieces to be welded, and back to the accumu- lators. Fifty cells are usually employed, and, if two boiler plates of about -j^- inch thick are to be united, the tool carries a very * NOTE.— Only strictly correct in the Thomson process, where energy absorbed is due to true resistance. The Benardos process uses the we, and energy is required to produce light, vir., to volatilise the carbon and render it incandescent : amovmting roughly to 30 volts in addition.